1. Field of the Invention
The present invention relates to a semiconductor optical device for use in optical communications and the like, more particularly to a semiconductor optical device of a high mesa ridge structure using a semi-insulating substrate.
2. Description of the Related Art
FIG. 10 is a perspective view of a conventional semiconductor optical modulator device, indicated in its entirety as 700.
The semiconductor optical modulator device 700 includes semi-insulating InP substrate 1. On the substrate 1, an n-InP lower clad layer 2, an InGaAsP light absorption layer 3, a p-InP upper clad layer 4 and a p-InGaAs contact layer 5 are successively laminated one on top of another. The InGaAsP light absorption layer 3 can be a layer of single composition or have a quantum well structure.
From the contact layer 5, the uppermost layer, to the half way of the lower clad layer 2, a ridge 10 is formed by mesa etching (high mesa ridge structure). The lower clad 2 is left on the substrate 1 on one side of the ridge 10.
Provided on the contact layer 5 of the ridge is a Ti/Au anode electrode 6. Provided on the lower clad layer 2 is a Ti/Au cathode electrode 7.
In the semiconductor optical modulator device 700, an incident light 21 enters into the vicinity of the light absorption layer 3 from one end face (on the incident side) and goes out of the other end face (on the exit side) as an exit light 22. When voltage (a modulated electrical signal) is applied between the anode electrode 6 and the cathode electrode 7 in the semiconductor optical modulator device 700, the light absorption coefficient of the light absorption layer 3 of a semiconductor layer varies according to the Franz-Keldysh effect or the Quantum Confined Stark Effect. As a result, the exit light 22 with modulated light intensity is produced at the end face on the exit side.
In order to sufficiently reduce the ohmic junction resistance of cathode electrode 7, the carrier density of the n-InP lower clad layer 2 must be 7×1018 cm−3 or larger.
However, as seen from the following expression (1), the larger the carrier density, the larger the light absorption coefficient (free carrier loss) of the semiconductor (H. C. Casey, Jr. and M. B. Panish “Heterostructure Lasers”, Academic Press, (1978), Chapter 3).αfc(cm−1)≈(3×10−18)·n+(7×10−18)·p  Expression (1)                αfc: free carrier loss        n and p: densities of electrons and positive holes (unit: cm−3)        
This fact has posed a problem that the larger the carrier density of the lower clad layer 2, the smaller the intensity of the exit light 22 going out of the semiconductor optical modulator device 700.